1. The Field of the Invention
The present invention is related to methods and apparatus for controlling the movement of materials exhibiting different physical properties by the application of acoustical energy to the materials. More particularly, the present invention is directed to methods and apparatus capable of continuously separating various materials from a fluid flow system when the materials exhibit physical properties, such as acoustical properties, different than the fluid medium.
2. The Prior Art
Numerous fields of modern technology require that materials which are being carried by a fluid system be separated from the liquid. For example, many industrial processes generate waste water which is contaminated by particulate matter. Separation of the particulate matter from the fluid allows the water to be easily disposed of and the particulate matter, if valuable, put to a good use. Furthermore, it is often desirable to separate an immiscible liquid or undissolved gas from a liquid.
The number of occasions in which it is necessary to separate particulates from a fluid medium is so pervasive that an extraordinary amount of attention has been devoted to the development of methods and apparatus to effect such separations.
One of the most rudimentary, yet pervasive, of separation techniques involves simple sedimentation. Sedimentation is the natural settling process wherein the particulates, gas bubbles, or immiscible liquids are separated due to gravitational force. The medium may then be removed by decanting or suction, while taking care not to disturb the particulates which have settled out of the medium.
Sedimentation techniques have the advantage of being simple and inexpensive. Unfortunately, the characteristics of the medium and the particles to be separated are often such that the time required for separation by sedimentation can be so long as to make this technique entirely impractical. Furthermore, if the particles are of a very small size, the particles will never "settle out" due to the Brownian motion of the molecules. Still further, if the carrying liquid is not kept free of any turbulence until sedimentation is complete, the particles will become resuspended. As a result, simple sedimentation techniques are practical only in certain limited situations.
In recognition of the fact that gravitational forces are too weak to effect rapid sedimentation in many instances, a frequent approach utilized in the prior art in order to increase the sedimentation rate of the material is to increase the gravitational force. This may be accomplished by subjecting the particle and medium mixture to centrifugation.
Centrifugation is a technique in which a container holding the particle and medium mixture is spun about a central axis in order to create centrifugal forces extending radially from the central axis. Increasing the speed of rotation will increase the centrifugal force applied to suspended particles, thereby increasing the rate of sedimentation. Modern centrifuges are capable of generating forces many thousands of times greater than gravity.
Yet another general technique used to separate some types of particles from a medium is filtration. Filtration involves the use of a porous filter that allows passage of the medium, while forming a barrier to the particles to be separated out. The speed of filtration can be enhanced by the application of pressure. However, the speed of filtration markedly decreases as a layer of filtered material builds up against the filter. For optimum performance, the filter must be replaced or cleaned frequently.
Each of the foregoing techniques is widely practiced and is extremely useful in many applications. Yet, each technique suffers significant drawbacks which limits its application to many situations.
For example, as mentioned above, gravitational sedimentation is not effective in many instances when the particles or the medium exhibit particular characteristics, such as when the medium is extremely viscous. Although centrifugation often speeds up the process of separation in such cases, centrifugation is often not completely effective; moreover, centrifugation is ill suited either for processing large quantities of a medium and particle mixture or for processing in continuous flow systems.
Filter techniques also suffer ineffectiveness when the particles to be separated from the medium begin to significantly build up on the filter. This build-up, or "caking", reduces the efficiency of the filter; at some point in the filtration process, this caking may completely stop the flow of the medium through the filter. If additional pressure is applied to the medium in order to improve the flow through the filter, damage to some types of separated material, e.g., blood cells, may occur.
Furthermore, filtration is generally ineffective when separating two immiscible liquids or when separating undissolved gases from a liquid. Some additional shortcomings of these traditional approaches may be better appreciated by reference to certain specific applications.
One area in which it is important to separate particles from a medium is in the medical arts. Numerous medical treatments and diagnostic tests, for example, require that blood (or other body fluids) be separated into their particulate and liquid components. Centrifugation has long been used for processing small amounts of blood in test tube sized containers. Such containers are typically filled with blood and placed in a small centrifuge, and then spun so that the blood cells accumulate in one portion of the container, leaving plasma in the upper portion of the container. The plasma is then decanted or suctioned off.
It will be readily appreciated that the use of test tube-sized containers is not very practical when a large amount of blood is to be separated into its plasma and cellular components. Yet, several medical procedures require separation of substantial volumes of blood into the cellular and plasma components.
One such procedure, generally known as "plasma phoresis", involves replacement of most of a patient's plasma with donor plasma or other suitable plasma substitute. This procedure involves removing whole blood from a patient, separating the cellular components from the plasma, discarding the plasma, and resuspending the cellular components in donor plasma. The reconstituted blood is then returned to the patient. Plasma exchange therapy has been successfully used to treat a variety of clinical conditions such as toxemias, drug overdoses, certain types of cancer, rheumatoid arthritis, and disseminated intravascular coagulation.
One attempt to improve the usefulness of centrifugation for use in plasma phoresis has lead to the development of continuous flow centrifuges. Unfortunately, continuous flow centrifuge processes also have serious drawbacks.
For example, the equipment necessary to perform continuous centrifugation is large, bulky, and also relatively expensive. Further, continuous centrifuges require relatively large volumes of blood to operate properly, and blood passing therethrough has a substantial residence time. This characteristic, in turn, mean that the patient must either do without a substantial volume of blood for an extended period of time, or must be provided with a whole blood substitute. Use of a whole blood substitute dilutes the patient's blood, and thus partially negates the aim of plasma phoresis to replace plasma, but not to replace the cellular components of the patient's blood.
Yet another disadvantage when using centrifugation to separate plasma from cellular blood components is that centrifugation causes the cellular components to become very tightly packed which may in itself cause damage to the blood cells. Subsequent reconstitution to whole blood by the addition of donor plasma is difficult to accomplish without causing hemolysis (i.e., damage) of the relatively delicate red blood cells. In any procedure in which biological materials are to be separated for reuse, extreme care must be taken so that the biological materials to be separated are not damaged by the process.
Another example of an area in which it is commonly important to separate another material from a medium involves petroleum-based materials. Oftentimes, a petroleum based product, hereinafter generally referred to as "oil," will be introduced into water during a processing step.
For example, in order to retrieve the maximum amount of oil possible from a particular amount of oil shale (rock having a high oil content), high temperature steam will be applied to the shale so as to extract the oil out from the nonpetroleum substances.
After the process is completed, the condensed steam contains a significant percentage of the oil that has been extracted from the oil shale. Since oil and water are immiscible, these liquids might be separated by the use of sedimentation or centrifugation. However, the same difficulties that were mentioned above are compounded when sedimentation or centrifugation are used to separate two immiscible liquids.
Another example of an area in which there is a need to separate material from the medium is liquid purification. Many times a liquid must be "purified" before it is used. While many applications do not require a degree of purification that is available when distillation purification procedures are used, many applications require that a significant amount of particulate matter be removed from the liquid.
In many applications, this particulate matter will be microscopic-sized particles of dirt. Removal of these dirt particles by sedimentation is impractical for the reasons mentioned earlier.
Filtration techniques are often used to remove such microscopic sized particles of dirt. However, the use of conventional filters to remove particles requires that, as mentioned above, the filter be replaced or cleaned as the particles build up on the filter media. Removal or cleaning of filters is often a time-consuming procedure requiring that the processing of the fluid be discontinued.
Because of the limitations of conventional techniques for separating particles from a medium, a great deal of effort has been directed to developing new techniques as well as improving the conventional techniques. One technique of relatively recent origin is shown in U.S. Pat. No. 4,055,491 issued to Porath-Furedi.
According to the Porath-Furedi patent, ultrasonic standing waves are used to cause flocculation of small particles, such as blood or algae, so that they will settle out of the carrying liquid. The Porath-Furedi patent describes a separation process which submerges an ultrasonic wave generator within a liquid having particles suspended therein and energizing it so that standing wave is established.
The establishment of a standing wave in the medium results in formation of pressure nodes to which the particles tend to migrate; these nodes and antinodes are at right angles to the direction of propagation of the ultrasonic waves, and the nodes are spaced from adjacent nodes by a distance equal to one-half of the wavelength of the ultrasonic wave.
The Porath-Furedi patent utilizes the accumulation of solid particles at the nodes or antinodes to cause flocculation, thereby assisting in simple gravitational sedimentation of the suspended particles when the ultrasonic standing wave is discontinued.
While the use of ultrasonic waves to flocculate particles as disclosed by the Porath-Furedi patent does substantially increase the sedimentation rate of those particles, the process is still quite slow. It also appears that the Porath-Furedi process is limited to intermittent flow "batch" operations. In particular, this process would not be practical in a high volume, or relatively rapid flow, process because of the extended residence time in the device that would be required to remove all of the particulate matter.
A variation of the Porath-Furedi process appears in U.S. Pat. No. 4,398,925 to Trinh et al. relating to the removal of air bubbles from a liquid, such as molten glass. The Trinh et al. process involves application of a particular ultrasonic frequency capable of establishing a standing wave having a single pressure well at a location half way between the bottom and the top of the container of liquid. Bubbles suspended in the liquid are pushed toward the pressure well, where they coalesce to form larger bubbles.
The ultrasonic wave is then interrupted so that the bubbles begin to float upward due to their buoyancy. After the coalesced bubbles have risen above the level of the pressure well, a second ultrasonic frequency is applied so that a second standing wave pattern is established--the second standing wave pattern having two pressure wells. The bubbles are then urged upwardly to the closest of the two pressure wells.
The foregoing process is then repeated. After the bubbles reach the upper pressure well, the ultrasonic generator is switched off so that bubbles continue to rise above the level of that well, and then yet a third ultrasonic frequency is applied, this one having three pressure wells. Again, the bubbles will be urged toward the highest pressure well, at which point the process can be repeated with progressively higher ultrasonic frequencies.
It will be readily appreciated that the Trinh et al. process relies on the buoyancy of the suspended bubbles to move the bubbles between wells during periods when the ultrasonic generator is switched off. Failure of the particles to move beyond the well will result in splitting of the particles and formation of multiple bands. Additionally, as with the Porath-Furedi process, it appears that the Trinh et al. process is primarily a batch process and is not well suited for use in situations such as plasma phoresis where a continuous supply of a medium must be subjected to the process.
Ultrasonic processes also have application in other fluid processing situations. For example, U.S. Pat. No. 4,013,552, issued to Creuter, shows the use of ultrasonic energy transmitted through sewage in order to reduce the size of the particles in the liquid by cavitation. Such cavitation enhances the ability of the particles to be exposed to oxygen and thus accelerate the action of aerobic bacteria. (The term "cavitation" refers to the creation of disturbances in a fluid caused by formation of gas bubbles by the application of acoustic energy.)
U.S. Pat. No. 4,346,011, issued to Brownstein, discloses a process which utilizes ultrasonic waves to flocculate particulate matter so as to prevent the particles from fouling a filter screen. The Brownstein patent, similar to the Creuter patent, appears to use cavitation to achieve its desired result.
In view of the foregoing, it will be appreciated that it would be a significant advancement in the art if methods and apparatus could be provided which are capable of effecting movement and separation of particles from liquids, immiscible liquids from each other, and undissolved gases from a liquid, that avoided the disadvantages of the techniques found in the prior art. It would also be of particular significance if methods and apparatus could be provided which have a high volume throughput, a relatively short residence time, and the ability to effect movement and rapid separation of the particles from the medium.
It would also be a significant improvement in the art to provide methods and apparatus for separating two materials without requiring physical contact with the materials and without causing significant damage to the materials, for example, blood. Furthermore, providing methods and apparatus for controllably moving, agitating, or separating materials of different physical properties, such as size or density, as well as methods which are adaptable to either batch mode or continuous flow systems, would be an important advancement in the art.